1. Field of the Invention
A method for conducting laser absorption measurements in high temperature process streams having high levels of particulate matter is disclosed. An impinger is positioned substantially parallel to a laser beam propagation path and at upstream position relative to the laser beam. Beam shielding pipes shield the beam from the surrounding environment. Measurement is conducted only in the gap between the two shielding pipes where the beam propagates through the process gas. The impinger facilitates reduced particle presence in the measurement beam, resulting in improved SNR (signal-to-noise) and improved sensitivity and dynamic range of the measurement.
2. Description of Prior Art
Monitoring the off-gas composition from combustion processes is useful for determining the extent of combustion taking place, conducting mass and energy audits, and monitoring the level of pollutants generated. Monitoring the gas composition is particularly useful for process control to perform corrective and/or optimization adjustments on selected process parameters. Identifying which process parameters to adjust is dependent on the parameter and the process. For example, processes showing an excess of CO in the exhaust are exhibiting incomplete combustion and a potential loss of energy. Optimizing the process would require an increase in the oxidant input or a decrease in the fuel input. Alternatively, other parameters can be adjusted, such as the process pressure that will affect the level of air infiltration into the process.
Many industrial processes, such as electric arc furnaces (EAF), waste incineration, and gasification, pose a special problem for off-gas composition due to the high gas temperature and the level of particles present. Conventional extractive gas sampling techniques using a water-cooled probe, to withdraw a sample from the process, followed by conditioning, using a chiller and filter to remove moisture and particles, suffer from probe plugging and corrosion. Probe plugging and corrosion problems can place a high maintenance demand on an analysis system, making it unattractive for routine use. In addition, extractive gas monitoring suffers slow response times that can be several tens of seconds to minutes to record the gas analysis result. Slow response time becomes a significant factor on dynamic processes such as an EAF, where conditions can be changing at a rate faster the diagnostic response, resulting in delays for applying process control. The combination of probe maintenance and slow response time issues make the technique unattractive on extremely harsh combustion systems.
In situ measurements using optical techniques, such as diode lasers, have been demonstrated on harsh combustion processes. In this case, a diode laser is tuned at the frequency of an absorption transition of the molecule of interest. A number of examples in the literature demonstrate the use of diode lasers to monitor the exhaust gas from a process.
For secondary steel processing, EAF's are used in a batch mode to melt the steel. These processes are particularly harsh, with exhaust gas temperatures less than about 1600 C and particle densities of 10 g/Nm3 to 100 g/Nm3. Stantec and Air Liquide have demonstrated extractive sampling monitoring of the off-gas, but probe maintenance often limits the usefulness and duration of conducting continuous day-to-day monitoring of the processes.
Sandia National Laboratory demonstrated analysis of the off-gas using diode lasers operating in the mid-IR working with the American Iron and Steel Institute. Their patent publication WO 99/26058 discloses the use of a diode laser propagating through the gap between the furnace exhaust and the main exhaust to measure CO, CO2 and H2O. However, working in the mid-IR range requires cryogenic cooling of the laser, and the system is not compatible with fiber optic components. The laser beam propagates across the whole length of the exhaust, resulting in an average measurement of the gas concentration along the beam path. In addition, the exhaust region of the EAF is affected by air being entrained into the flow along the gap. This air entrainment is used to cool the exhaust gas and burn off any residual CO from the process itself. Therefore, conducting a path-averaged measurement at the gap position will lead to error, due to the uncertainty in the true path length of the measurement. Additionally, the combination of the long pathlength through the duct (typically four to eight feet long) and particle densities results in severe laser beam attenuation. The combination of these two effects can be detrimental to the measurement and place limitations on the accuracy of measured concentration values.
Similarly, a conceptual layout using near-IR lasers for CO and H2O monitoring instead of mid-IR with beam propagating across the duct, without any shielding, has been shown. For both mid-IR and near-IR measurements, shielding the beam from the ambient atmosphere is not required, since CO is present only in the EAF gap area and the H2O lines are only present at high temperatures.
Inclusion of O2 monitoring in the near-IR on an EAF as demonstrated by Dietrich et al. does require shielding the beam from the ambient air. Dietrich, et al., LASER ANALYSIS OF CO AND OXYGEN IN EAF OFF-GAS, 59th Electric Furnace Conference and 19th Process Technology Conference Proceedings, Iron and Steel Society (2001). In this case, shielding pipes are used on each side of the furnace gap to launch and receive the beam for monitoring O2 and CO. Placement of the shielding pipes in the EAF exhaust sets the pathlength to the desired distance to reduce the effect of particles and remove uncertainties regarding the pathlength. Despite the shortened path length, difficulties in obtaining adequate light transmission persisted due to the high particulate levels present. Similarly Thomson, et al., shows a conceptual application using a near-IR laser system. Thomson, et al., LASER BASED OPTICAL MEASUREMENTS OF ELECTRIC ARC FURNACE OFF-GAS FOR POLLUTION CONTROL AND ENERGY EFFICIENCY, Innovative Technologies for Steel and Other Materials, Met. Soc., The Conference of Metallurgists, Toronto (August 2000).
Thus, a problem associated with methods for gas monitoring in high particle density flow streams is that they provide a slow response time and thereby do not adequately indicate process conditions to enable optimal process control.
Still another problem associated with methods for gas monitoring in high particle density flow streams that precede the present invention is that they do not facilitate optimal use of the potential energy present in the process by-products for heat generation.
Yet another problem associated with methods for gas monitoring in high particle density flow streams that precede the present invention is that they are susceptible to probe plugging and corrosion.
An even further problem associated with methods for gas monitoring in high particle density flow streams that precede the present invention is that they require undue replacement of the monitoring equipment.
Still a further problem associated with methods for gas monitoring in high particle density flow streams that precede the present invention is that they do not provide continuous, near real-time measurements of the species concentration in the waste gases, with acceptable accuracy, so as to facilitate an adapted dynamic monitoring of process characteristics.
Another problem associated with methods for gas monitoring in high particle density flow streams that precede the present invention is that they do not adequately facilitate control of the CO and H2 combustion by-products.
Yet another problem associated with methods for gas monitoring in high particle density flow streams is that they are not readily adaptable for use in industrial processes that experiences high particle densities, temperature gradients, mechanical vibration, rapid variations in temperature and gas composition, and high radiation loads from the process itself.
Another problem associated with methods for gas monitoring in high particle density flow streams is that they do not adequately provide continuous monitoring of the process conditions.
Still another problem associated with methods for gas monitoring in high particle density flow streams is that they cannot be adapted readily for use in high temperature environments and still provide a durable method that does not require unduly frequent replacement of equipment.
In contrast to the foregoing, the present invention provides a enhanced gas monitoring in high particle density flow streams that seeks to overcome the foregoing problems and provide a more simplistic, more easily constructed and relatively reliable methodology.
For the foregoing reasons, there has been defined a long felt and unsolved need for a method for enhanced gas monitoring in high particle density flow streams that seeks to overcome the problems discussed above, while at the same time providing a simple, easily constructed and maintained design that facilitates more reliable process control.